1. Field of the Invention
The present invention relates to a conductive-pattern recognition apparatus and more particularly, to conductive-pattern recognition apparatus for recognizing a conductive pattern formed on a substrate, which is applicable to automatic pattern inspection of Printed Wiring Boards (PWBs) or the like.
2. Description of the Prior Art
In recent years, automation in the fabrication processes of PWBs has been progressing more and more for the purpose of labor-saving andefficiency-raising. According to this tendency, various inspection procedures such as visual inspection and production-lot discrimination, which were performed manually, have been becoming automated.
For example, various conductive-pattern recognition apparatuses making it possible to perform automatically the pattern inspection procedures of PWBs have been developed and practically used.
An example of the conventional conductive-pattern recognition apparatuses is shown in FIG. 1, which recognizes optically the conductive patterns of PWBS.
As shown in FIG. 1, this conventional conductive-pattern recognition apparatus is comprised of a moving or translation mechanism 106 for moving or translating a PWB 109 to be inspected in a direction A, a strip-shaped light source 130 for illuminating the surface of the PWB 109 with light, a lens 131 for collecting the light reflected by the surface of the PWB 109, and an optical line sensor 132 for sensing the light reflected by the surface of the PWB 109 and collected by the lens 131. The surface of the PWB 109 is covered with an insulator 133 except for a conductive circuit pattern 110. The optical line sensor 132, which has Charge-Coupled Devices (CCDs) arranged along a straight line, is fixed in parallel to the light source 130 and the lens 131.
On recognition or operation of this apparatus, the PWB 109 with the conductive pattern 110 is translated by the translation mechanism 106 in the direction A perpendicular to the line sensor 132. The light emitted from the light source 130 is reflected by the surface of the PWB 109 and then, the reflected light is collected by the lens 131 to be inputted into the line sensor 132.
The intensity of the reflected light by the conductive pattern 110 is higher than that of the reflected light by the insulator 133. Therefore, the conductive pattern 110 is able to be recognized by detecting the intensity difference of the reflected light.
In addition to the optical recognition apparatus explained above, pressure-sensitive recognition apparatuses have been known. An example of these conventional pressure-sensitive recognition apparatuses is shown in FIG. 2, which is an embossed-character reader and is disclosed in the Japanese Non-Examined Patent Publication No. 2-257380 published in 1990.
The conventional embossed-character reader is comprised of an idler roller 206 for moving or translating an embossed card 234 (i.e., a specimen) on which embossed characters 235 are formed, a pressure-sensitive rubber roller 236 whose electric resistance varies according to an applied pressure, a rotary encoder 237 fixed to the axis of the pressure-sensitive rubber roller 236 for detecting the rotational amount or distance of the roller 236, and a resistance detection means (not shown) for detecting the electric resistance of the pressure-sensitive rubber roller 236.
The pressure-sensitive rubber roller 236 has a pressure-sensitive rubber sheet formed by a silicone rubber containing proper conductive particles uniformly dispersed therein. The pressure-sensitive rubber sheet has electrodes S11, S12, S13, and S14 at its four sides.
In the conventional embossed-character reader shown in FIG. 2, the specimen or embossed card 234 is translated in the direction A between the rollers 206 and 236 on reading operation. During the reading operation, the electric resistance of the pressure-sensitive rubber sheet of the roller 236 and the rotational distance of the roller 236 are detected, thereby obtaining a pattern of the area change due to the embossed characters 235. Then, the pattern of the area change thus obtained is compared with the specific reference patterns prepared in advance, thereby recognizing the embossed characters 235 on the card 234 using the pattern matching procedure.
For example, if the embossed characters 235 are a series of the characters "0123" as shown in FIG. 3, the area change of these characters 235 along the translation or moving direction A is converted to the change of an electric current as shown in FIG. 4. This pattern of the electric current is compared with the reference patterns to thereby recognize the characters 235 through the pattern matching procedure. Thus, the characters 235 are found "0123".
With the conventional optical conductive-pattern recognition apparatus shown in FIG. 1, however, there is the following problem
Specifically, the conductive pattern 110 of the PWB 109 is typically made of copper (Cu) and the insulator 133 of the PWB 109 is made of an epoxy resin reinforced by a glass cloth. Therefore, the conductive pattern 110 has metallic luster allowing the illuminated light to be efficiently reflected. On the other hand, the insulator 133 having a rough surface causes irregular reflection and/or transmission of the illuminated light, resulting in reduction of reflection of the illuminated light.
Moreover, the conductive pattern 110 made of copper tends to be oxidized by salts or acids adhered on human hands and oxygen (O.sub.2) contained in the atmospheric air, thereby forming a thin film of copper oxide on the pattern 110. The thin film of copper oxide thus produced is difficult to reflect the illuminated light. As a result, the obtainable intensity difference of the light between the conductive pattern 110 and the insulator 133 becomes small and consequently, recognition error tends to occur.
There arises a similar problem when the conductive pattern 110 is made of a dark-colored material such as carbon paste or a conductor/resin mixture such as conductive paste. If things come to the worst, recognition of the pattern 110 will become impossible.
With the conventional pressure-sensitive conductive-pattern recognition apparatus shown in FIG. 2, there is a problem that the embossed characters 235 is unable to be recognized unless the characters 235 are protruded from their neighboring area. This is because the reading or recognition operation is performed by detecting the change of the electric resistance of the pressure-sensitive rubber roller 236.
Also, there is another problem that recognition is possible for simple patterns only due to the following reason.
It is popular that the conductive patterns of PWBs are complex compared with the embossed characters 235. Therefore, the change of the electric resistance obtained from the conductive patterns of PWBs is difficult to be pattern-matched with the reference patterns. Further, the reference patterns themselves are extremely difficult to be prepared in advance.